An optical fiber (i.e., a glass fiber typically surrounded by one or more coating layers) conventionally includes an optical fiber core, which transmits and/or amplifies an optical signal, and an optical cladding, which confines the optical signal within the core. Accordingly, the refractive index of the core n, is typically greater than the refractive index of the optical cladding ng (i.e., nc>ng).
For optical fibers, the refractive index profile is generally classified according to the graphical appearance of the function that associates the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's outer cladding (e.g., an outer optical cladding) is shown on the y-axis. The refractive index profile is referred to as a “step” profile, a “trapezoidal” profile, a “parabolic” profile, or a “triangular” profile for graphs having the respective shapes of a step, a trapezoid, a parabola, or a triangle. These curves are generally representative of the optical fiber's theoretical or set profile. Constraints in the manufacture of the optical fiber, however, may result in a slightly different actual profile.
Generally speaking, two main categories of optical fibers exist: multimode fibers and single-mode fibers. In a multimode optical fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber. In a single-mode optical fiber, the signal propagates in a fundamental LP01 mode that is guided in the optical-fiber core, while the higher order modes (e.g., the LP11 mode) are strongly attenuated. The typical diameter of a single-mode or multimode glass fiber is 125 microns. The core of a multimode optical fiber typically has a diameter of between about 50 microns and 62.5 microns, whereas the core of a single-mode optical fiber typically has a diameter of between about 6 microns and 9 microns. Multimode systems are generally less expensive than single-mode systems, because multimode light sources, connectors, and maintenance can be obtained at a lower cost.
For the same propagation medium (i.e., in a step-index multimode optical fiber), the different modes have different group delay times. This difference in group delay times results in a time lag (i.e., a delay) between the pulses propagating along different radial offsets of the optical fiber. This delay causes a broadening of the resulting light pulse. Broadening of the light pulse increases the risk of the pulse being superimposed onto a trailing pulse, which reduces the bandwidth (i.e., data rate) supported by the optical fiber. The bandwidth, therefore, is linked to the group delay time of the optical modes propagating in the multimode core of the optical fiber. Thus, to guarantee a broad bandwidth, it is desirable for the group delay times of all the modes to be identical. Stated differently, the intermodal dispersion should be zero, or at least minimized, for a given wavelength.
To reduce intermodal dispersion, the multimode optical fibers used in telecommunications generally have a core with a refractive index that decreases progressively from the center of the optical fiber to its interface with a cladding (i.e., an “alpha” core profile). Such an optical fiber has been used for a number of years, and its characteristics have been described in “Multimode Theory of Graded-Core Fibers” by D. Gloge et al., Bell system Technical Journal 1973, pp. 1563-1578, and summarized in “Comprehensive Theory of Dispersion in Graded-Index Optical Fibers” by G. Yabre, Journal of Lightwave Technology, February 2000, Vol. 18, No. 2, pp. 166-177. Each of the above-referenced articles is hereby incorporated by reference in its entirety.
A graded-index profile (i.e., an alpha-index profile) can be described by a relationship between the refractive index value n and the distance r from the center of the optical fiber according to the following equation:
  n  =            n      max        ⁢                  1        -                  2          ⁢                                    Δ              ⁡                              (                                  r                                      r                    1                                                  )                                      α                              
wherein,
α≧1, and α is a non-dimensional parameter that is indicative of the shape of the index profile;
nmax is the maximum refractive index of the optical fiber's core;
r1 is the radius of the optical fiber's core; and
  Δ  =            (                        n          max          2                -                  n          min          2                    )              2      max      2      
where nmin is the minimum refractive index of the multimode core, which may correspond to the refractive index of the outer cladding (most often made of silica).
A multimode optical fiber with a graded index (i.e., an alpha profile) therefore has a core profile with a rotational symmetry such that along any radial direction of the optical fiber the value of the refractive index decreases continuously from the center of the optical fiber's core to its periphery. When a multimode light signal propagates in such a graded-index core, the different optical modes experience differing propagation mediums (i.e., because of the varying refractive indices). This, in turn, affects the propagation speed of each optical mode differently. Thus, by adjusting the value of the parameter α, it is possible to obtain a group delay time that is nearly equal for all of the modes. Stated differently, the refractive index profile can be modified to reduce or even eliminate intermodal dispersion. Furthermore, the value of the parameter αmay be adjusted to provide a high bandwidth at a given wavelength or over a range of wavelengths (e.g., between about 850 nanometers and 1300 nanometers).
Multimode optical fibers have been the subject of international standardization under the ITU-T G.651.1 recommendations, which, in particular, define criteria (e.g., bandwidth, numerical aperture, and core diameter) that relate to the requirements for optical fiber compatibility. The ITU-T G.651.1 standard (July 2007) is hereby incorporated by reference in its entirety.
In addition, the OM3 standard has been adopted to meet the demands of high-bandwidth applications (i.e., a data rate higher than 1 GbE) over long distances (i.e., distances greater than 300 meters). The OM3 standard is hereby incorporated by reference in its entirety. With the development of high-bandwidth applications, the average core diameter for multimode optical fibers has been reduced from 62.5 microns to 50 microns. Multimode optical fibers are commonly used for short-distance applications requiring a broad bandwidth, such as local networks or LAN (local area network).
Commonly owned European Publication No. 2,312,350 (and its counterpart U.S. Patent Publication No. 2011/0085770), each of which is hereby incorporated by reference in its entirety, proposes to add a trench in the inner depressed cladding to reduce leakage losses and to bring the outer cladding closer to the core. This application, however, concerns single-mode optical fibers, which present different design-problems. For example, only LP01, LP11 and LP02 modes need to be managed without constraints on bandwidth, core size, and numerical aperture. Furthermore, the ratio of the “depressed cladding's outer radius” to the “central core's outer radius” is much larger than in a multimode refractive index profile.
International Publication No. 2008/085851 (and its counterpart U.S. Patent Publication No. 2008/0166094), each of which is hereby incorporated by reference in its entirety, describe multimode optical fibers having a graded-index central core surrounded by a depressed-index annular portion between the central core and the optical cladding. This depressed-index annular portion is added to reduce the bending losses of a graded-index multimode optical fiber. Generally-speaking, the solutions disclosed in International Publication No. 2008/085851 and U.S. Patent Publication No. 2008/0166094 do not concern depressed graded-index multimode optical fibers. Applying the proposed solutions to fluorine-only-doped profiles does not permit the reduction of leakage losses when the cladding is composed of a depressed cladding and an outer cladding made of natural silica.
European Publication No. 2,166,386 (and its counterpart U.S. Patent Publication No. 2010/0067858), each of which is hereby incorporated by reference in its entirety, describe multimode optical fibers having a graded-index central core, an inner cladding adjacent to the graded-index central core, and a surrounding depressed-index annular portion between the central core and the optical cladding. This depressed-index annular portion is added to reduce the bending losses of a graded-index multimode optical fiber. Generally-speaking, the embodiments disclosed in European Publication No. 2,166,386 and U.S. Patent Publication No. 2010/0067858 A1 do not concern depressed graded-index multimode optical fibers. Applying the proposed embodiments to fluorine-only-doped profiles does not permit the reduction of leakage losses when the cladding is composed of a depressed cladding and an outer cladding made of natural silica.
European Publication No. 2,220,524, which is hereby incorporated by reference in its entirety, describes multimode optical fibers having a graded-index central core, an inner cladding adjacent to the graded-index central core, and a surrounding depressed-index annular portion between the central core and the optical cladding. This depressed-index annular portion is added to reduce the bending losses of a graded-index multimode fiber. Generally-speaking, the optical fibers disclosed in European Publication No. 2,220,524, do not concern depressed graded-index multimode optical fibers. Applying the proposed embodiments to fluorine-only-doped profiles does not permit the reduction of leakage losses when the cladding is composed of a depressed cladding and an outer cladding made of natural silica.
Moreover, the inclusion of a depressed trench results in the appearance of additional leaky modes that will co-propagate with the desired leaky modes. These additional leaky modes have effective refractive indices that are lower than those sustained by the depressed graded-index central core. As compared with the numerical aperture measured in the graded-index fibers without a depressed trench according to the well-known, standardized IEC 60793-1-43 method, the lower effective refractive indices corresponding to these additional leaky modes lead to an increase in the numerical aperture measured on two meters by the far-field pattern in the graded-index optical fibers including a depressed trench. Numerical aperture of a graded-index multimode optical fiber (GIMMF) with a depressed trench can appear larger than would be expected based on the value of the parameter Δ of its graded-index core.
In addition, the leaky modes can contribute to the distortion of core size as measured under overfilled launch (OFL) from the near-field pattern at the output of a two-meter sample using the IEC 60793-1-20 Method C. Thus, the core size of a GIMMF with a depressed trench can appear larger than would be expected based on the value of its graded-index central core's width.
Distorted numerical aperture and core size measurements may lead to incorrect conclusions regarding the core size and the refractive index profile's value of the parameter Δ, which are important for connectivity. Core size and A determine the number and shape of the guided modes. Such a difference in the number and shape of the guided modes between two different graded-index optical fibers can lead to mode mismatching and therefore to high splice losses or connectivity losses.
In addition, multimode fibers that include (i) central cores having indices close to that of silica and (ii) down-doped index claddings provide several advantageous features compared with standard multimode fibers with up-doped central cores and silica claddings. For example, such optical fibers may exhibit improved radiation-resistance and hydrogen-resistance, as well as low losses and large bandwidths. These structures, however, are difficult and expensive to make because a highly depressed cladding is deposited. The deposited cladding's refractive index difference with respect to silica may reach −16×10−3. Thus, the PCVD technique is often used. This technique is an inside deposition process (like the methods MCVD and FCVD) that facilitates the achievement of such low, negative refractive index differences. That said, the silica substrate tube used in the PCVD technique must be positioned far from the central core to prevent the modes from leaking and thus experiencing high leakage losses. The constraints imposed by the positioning of the substrate tube typically yield small-diameter core rods, thereby increasing manufacturing costs.
Thus, there exists a need for a multimode optical fiber that includes a depressed cladding that limits the impact of the leaky modes on other optical-fiber characteristics (e.g., bandwidth, core size, and/or numerical aperture) but that may be deposited within a silica substrate tube at a reduced width.